Perhaps in 1790 that's all the census needed to know (that and how may slaves you owned), but it's a far different situation now. Socioeconomic and ethnic data is important in determining the types of services various areas need and plays an important part in know just who an "American" really is.
As an aside, the census had nothing to do with the Japanese internment during WWII. At most it made calculating the number of Japanese-Americans easier, allowing the round up to be more accurate. Maybe. Given how easy it is to separate people by obvious ethnic ancestry, the round up would have occurred any way. Besides which, it's not as if either of scenarios mentioned in the OP actually provided anything more than numbers. They didn't provide addresses, names, or any actual personal information. Merely the number who marked a certain ethnicity in a certain county.
So yes, these people are still just paranoid.